1. Field of the Invention
The present invention relates to a metal casing for a semiconductor device and method for manufacturing it, and more specifically to a metal casing suitable for a semiconductor device such as a photodiode, a laser diode, a microwave device for microwave communication, and a high power supply which has high thermal conductivity, high heat spreading and thermal expansion coefficient similar to those of a semiconductor, a ceramic or glass element disposed on the metal casing, and method for manufacturing the metal casing at low cost.
2. Description of Related Art
A metal casing for a high power supply and a semiconductor device used for optical communication or microwave communication generally comprises a base member on which the semiconductor device is mounted and an enclosure member which is fixed on the base member and which surrounds the semiconductor device and on which terminal pins for wiring are fixed by some particular ways. For example, terminal pins are mounted on ceramic mounting members and the ceramic mounting members are fixed to the enclosure member of the metal casing. In another case, terminal pins are fixed to holes of the enclosure member by using sealing glass.
In case of a metal casing for a semiconductor device for optical communication, the enclosure member comprises an opening for transmitting or receiving optical signals. Glass is usually fitted to the opening. An optical fiber is arranged near the outside of the opening.
In addition, a metal frame may be disposed on an upper edge of the enclosure member in order to fix a sealing cap.
The metal casing is heated while the semiconductor device mounted in it functions. As mentioned above, since the ceramic members and the glass part are fixed on the enclosure member, the enclosure member is preferably formed of a material having a thermal expansion coefficient similar to those of the ceramic members and the glass part. In addition, the enclosure member generally has a complicated shape, its material is required to have good machinability. Furthermore, the enclosure member should have certain rigidity.
In order to fulfill the above requirements, an enclosure member of a conventional metal casing is often formed of an iron-nickel alloy or an iron-nickel-cobalt alloy. On the contrary, a base member of the conventional metal casing is formed of a metal or an alloy having a good thermal conductivity and heat spreading such as copper or a copper-tungsten alloy in order to radiate heat generated by a semiconductor device mounted on it.
In the conventional metal casing, the enclosure member is jointed to the base member by brazing utilizing a silver-copper solder. However, since the enclosure member and the base member are formed of different metals or alloys, the conventional metal casing is liable to distort during the brazing. In particular warping of the base member is often caused.
If a semiconductor device for optical communication such as a laser diode or a photodiode is mounted in the distorted metal casing an optical coupling of the semiconductor device often deviates from that of an optical fiber so that a substantial optical power is decreased.
If a microwave semiconductor device is mounted in the distorted metal casing, the semiconductor device may be sometimes damaged or instability of a ground voltage and drop of heat radiation are caused so that the device becomes out of order.
In order to resolve the above problems, the base member of the metal casing is sometimes ground after the brazing so as to correct the warping. However, this work is of poor efficiency.
It can be considered that the metal casing is integrally formed of a base member and an enclosure member of an equal material in one-piece. In this case, the metal casing has been formed of a copper-tungsten alloy having a thermal expansion coefficient similar to those of some ceramic and glass materials, good thermal conductivity and heat spreading. However, in order to manufacture the metal casing integrally formed of the copper-tungsten alloy, it should be machined from a copper-tungsten alloy block. This results high cost and hard to conduct mass production.
The metal casing for a semiconductor device of a copper-tungsten alloy or a copper-polybdenum alloy is preferably manufactured by using powder metallurgical techniques such as one disclosed in Japanese Patent Application Laid-open No. 59-21032, in particular by sintering and infiltration. A metal injection molding process which is an improved sintering process is disclosed in International Patent Publication WO89/02803. In the metal injection molding process, copper powder and tungsten powder are mixed with an organic binder material to form an admixture. The admixture is molded by injection molding to form a predetermined green shape. The green shape is debinderized and sintered to produce a product.
However, the green shape should contract in volume equivalent to that of the binder included in it during the sintering so as to obtain a required density and thermal conductivity. In case of a product having a complicated shape such as a metal casing for a semiconductor device, the contraction does not uniformly occur, which is liable to cause distortion of the product so that it is difficult to obtained a high accuracy in shape.
In addition, the green shape includes 5 to 50 wt % copper powder, which melts and effuses to form a copper layer on a surface of the product so as to form a effused zone. The effused zone also spoils accuracies in shape and size. Therefore, the metal casing for a semiconductor device should be machined after the sintering in accordance the above prior art.